Category Archives: Guitar construction

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As I do with every component of a guitar, I want to understand the purpose of the back plate so I can build it in the best way I possibly can. Gore and Gilet’s book Contemporary Acoustic Guitar has been very useful to me (as always) by raising the possibility of building what they call a “live back” – in other words, a back plate that contributes to the overall tone of the instrument by vibrating along with the top when the strings are sounded. This adds a complexity to the tone that wouldn’t be there with a non-live back, so should be desirable.

I’m not convinced I have succeeded in this yet. In this page I’m going to show how I make and brace the back plate, and evaluate my current success in the quest for the live back. I have adopted the back brace plan used in Trevor Gore’s instruments as a starting point, as you’ll see. At the end of this entry I’ve put some evidence that indicates I haven’t breathed life into my backs just yet. Perhaps I need a thunderstorm and lots of electric arcs and an assistant called Igor.

But now I want to show you some test results that have got me thinking more about this concept.

What is the purpose of the back plate?

“Traditional” steel string guitar backs use a ladder bracing system that I also used in my early attempts. It looks like this:

Traditional ladder bracing

The purpose of the back braces has always been seen as mechanical rather than tonal. Thin panels tend not just to flex under load but to distort as they absorb and lose water. Wood always expands much more across the grain than along it, so this can lead to weird shape chances unless stiffly braced. The back centreline join can also be pulled apart by dimensional changes in the wood if the guitar is moved into a dry climate after being built in a humid one.

Many people argue that when you play the guitar your body is in contact with the back and damps out all the vibration anyway, so attempting a live back is pointless. This isn’t at all true, as I think I can show you.

The T(1,1)1 “breathing mode

A guitar’s simplest mode of vibration is called the “breathing” or T(1,1)1 mode. You can see it in this animation as Mode #2:

This simple mode would not exist if the back were not there. It’s as simple as that. The “Breathing” mode is low frequency (usually around 100Hz for a guitar) and contributes bass boom and depth to the sound.

But can the back contribute higher modes to the overall guitar sound as well? This is what we mean by “live back”.

Can I convincingly show you a live back in action? Sadly, no. But I live in hope…maybe this guitar will achieve it. Stay tuned!

What does the back actually do when a guitar is played?

This isn’t as simple a question as it sounds. Gore and Gilet’s analysis shows that a soundbox is made up of three separate resonators: the top, the back/sides, and the air contained in the box.

Each of these has its own fundamental frequency and an associated string of harmonics or partials, as does every resonator. Here, for example, is the frequency spectrum of a singing wineglass – the highest peak is the fundamental, and the other peaks represent the inevitable series of higher frequency partials produced by different modes of vibration.

Here is the string of a guitar string ringing. It behaves in a very similar way, although in this case we can call the partials “harmonics” because they have a simple mathematical relationship with the fundamental, which partials do not:

Where it all becomes complicated is when you start to join simple resonators together, as you do when assembling a guitar. Because they’re connected, each one influences the others, and not in a simple way.

Okay, enough physics for now. Let me show you what happens when you start to mess with one out of the three joined soundbox resonators. I’m going to use some data from tapping one of my Jumbo guitars to illustrate.

Firstly, here’s the spectrum produced by tapping the top with the guitar held up by the neck and no contact with the soundbox:

Jumbo top response held free from body

Notice that it isn’t a simple spectrum like that for the wineglass, although there are a number of peaks that stand out. Notice particularly the first one at around 100Hz, which is the total resonance of the top, back/sides and airbody (the T(1,1)1 or “breathing” mode we’ve already seen). Now look at what happens when I hold the guitar in my normal playing position resting lightly against me:

Jumbo top response in playing position

I have kept the original signature (the red line) so you can see the effect of body contact (the black line). Very little at all, wouldn’t you agree?

Okay – one last graph before we get on with construction details. This one shows what happens when I aggressively damp the back by pressing my hand firmly against it while tapping the top:

Jumbo top, back heavily damped

See the difference? Again, I have left the first freely-held response (red line) for comparison. The most striking effect of heavy back damping is the complete disappearance of the first peak in the response, the “breathing” mode. Immobilising the back has broken the link between the three resonators in a dramatic way. The tap sound was an anaemic thunk.

What does this tell us? Well, it makes clear a number of vital things about how a guitar works:

1. Messing with one of the three coupled resonators affects the other two, sometimes dramatically. This understanding opens up possibilities for tuning the soundbox if you know how.

2. The response with the guitar held in the playing position shows very little difference to the guitar held freely – the T(1,1)1 airbody peak is still strongly there. This suggests that the back continues to have a strong influence on sound production when playing, and that the common wisdom (that the player’s body contact completely damps the back and makes it useless to strive for a live back) is emphatically wrong.

3. The “breathing” mode is important to the production of sound by a guitar, and it isn’t there unless there is a back to contain it and importantly that it is free to respond to the strings. When tapping with the back aggressively damped, the tap sound lost its depth and became a rather pathetic thud. This is because with the back immobilised, the box can’t “breathe”.

To sum up:

a) the guitar back is a very important factor in determining the overall sound of the guitar, and deserves close design attention;

b) all guitar backs are “live” in the sense that they are essential for the guitar to produce a sound with any depth (damp it aggressively and the “breathing” mode dies);

c) I haven’t yet achieved the extra Gore/Gilet step of building a back that further enhances the sound by adding complexity through its own resonant response on top of the “breathing” effect.

Building the back

This is the bamboo guitar back nearly finished, showing the Gore-pattern ladder/radial bracing. Here I have been cutting the “gable” with a brace chisel (just visible bottom left). The back is resting is the 20ft radius dish to preserve the curvature as the braces are fitted.

The Gore back bracing design

These are the spruce brace blanks machined to size:

Brace blanks (back braces on the right)

The are the steps in putting a back together are:

1. thinning the panels to the right thickness;

2. joining the two panels to get enough width for a back plate;

3. bracing the back plate;

4. attaching the back plate to the sides.

Thinning the panels

I thin the back panels using a drum sander before joining them:

Drum sander

I check the thickness often with vernier calipers. A trick I have learned in using a drum sander is that you don’t need to adjust the drum height for each pass, especially when you’re nearing the thickness you want.

Vernier calipers

This way I can control the thickness to an accuracy of 0.1mm. In another page (The bamboo guitar – Part 3) I have explained how I chose what thickness I would work to for the bamboo. My aim was to produce a back with properties as close to my usual blackwood backs as I could.

Once the panels are the right thickness it’s time to join them together. Though initially it daunted me, it’s surprisingly easy to get a good invisible joint. Here’s a top I’ve joined using my method (the chalk marks help keep the two panels in the right relationship while I’m working on them):

A top joint

These are actually spruce top panels, but the principle is the same and I forgot to take photos for the bamboo back.

The first step is to plane the joining edges straight on a shooting board, then fitting them together on a flat surface to look for gaps. Using an old plane with sandpaper stuck to the sole, with care you can remove the high spots and get an invisible joint. The trick is to get one edge as straight as possible, then adjust the other to fit it. The shooting board allows you to keep the edges square as you work on them.

Joining the panels

Once you’ve got the edges fitting perfectly, it’s time to join them together. I use a simple but effective setup that I got from somewhere I can’t remember. I have a flat table with a batten fixed along each side, one of them adjustable so I can clamp different size pieces.

The jointing table

I slip a small batten under the the panels where they join so the edges are slightly lifted, and adjust the edge battens to just hold them in place:

To make sure the panels stay in vertical alignment, I hold the joint down with cork-backed blocks and go-bars.

When the batten is slipped out the edges are gently forced down to make a beautifully tight joint. Oh, and don’t forget to put glue on first, and put wax paper down so you don’t glue the panels to the table…

Once the glue is set, you can take the joined back plate out of the clamps, clean up any dried glue, and cut out the correct shape.

Adding the back braces

Time to get out the old 20ft sanding dish again, last seem while profiling the side assembly. This time it will be used for two things:

a) sanding the braces to the right curve;

b) acting as a mould to make sure the back conforms to the right curvature.

The first bracing piece is the marriage strip, a piece of 20 x 4mm spruce kept from past top panel production. It is glued along the centre joint to reinforce it, so it’s important to cut it so that its grain crosses the joint at right angles – you can see the strip across the bottom of this photo:

Marriage strip

The marriage strip is glued on and the whole back pressed into the dish using go-bars. Glue is slippery, so keep a sharp eye on the strip to make sure it doesn’t absent- mindedly wander off before the glue grabs. I also make sure that I know exactly where the marriage strip must end to meet the end blocks neatly once the back is glued to the sides.

The back brace blanks have already been machined to 20 x 10mm, keeping the grain vertical. I cut each one to length, mark out the scallops, then sand the bottom surfaces to the 20ft curve by rubbing them in the sanding dish, I the cut out the scalloping on my bandsaw and finish them with a small sanding drum in my drill press.

I use a sharp X-acto knife to cut through the marriage strip and a sharp chisel to remove wood so the braces can be glued in.

Here’s how it looks with the go-bars in place:

Braces held in place with go-bars

Next to go in are the radial tone bars:

The scalloping of the braces reduces the mass of the back, but importantly also allows some later tuning of the back resonances once the guitar is finished. (Remember we’ve found that messing with one of the three resonators will affect both the others.) The general principle is that the thicker the braces the higher the fundamental resonance of the back plate will be. If I want to lower the back resonances later, I can carefully thin down the central scallops working through the soundhole.

By the way, the idea that you can tune each brace separately doesn’t work in practice (or even in theory, for that matter – sorry Roger Siminoff) because the back responds as a whole, not a simple sum of its individual parts. Maybe there’s a saying in that somewhere.

Finally, I use a very sharp chisel to carve the top edge of each brace at an angle so it comes to a point like a gabled roof. This reduces their mass but pretty much preserves their elasticity.

Joining the back to the sides

The sides of the soundbox have already been shaped to match the curvature of the back plate (see The bamboo guitar – Part 6) so it’s now a matter of joining the two assemblies. Because I don’t use an edge binding to hide the joint, I need it to be a perfect fit and firmly clamped:

Joining the back to the sides

I use cylindrical screw clamps from Stewart-MacDonald (www.stewmac.com). Once the glue has set, I can carefully trim the overlap down and sand the edge so it is smooth and slightly rounded off.

I now put on a coat or two of epoxy to seal and harden up the grain, leaving the guitar looking like this:

Epoxy sealing coats on

Not bad for bamboo, wouldn’t you say?

And now we return to the live back issue…

So how will I know if I succeed in producing a live back? The evidence would be there if the spectral signature of the back, or at least parts of it, showed up clearly in the overall signature of the guitar. This sounds simple, but actually isn’t.

We’ve seen that the three resonators, the airbody, the top, and the sides/back that make up the soundbox of a guitar, link together as a whole but not in a simple way – each one changes the others in subtle ways. So if they’re linked so intimately, how can we sort out if any part of the overall sound (other than the breathing mode) comes from the back specifically? Can we isolate what the back is up to and compare it to the overall tonal signature?

There is a way to decouple the top signature from the back signature: block up the soundhole. This takes the airbody out of the mix because it can’t breath, leaving the top and back much less intimately connected.

Here is a comparison of the overall tonal signature of the guitar (the top tapped with the soundhole open) – the red line – in the normal playing position. The blue line is as close as we’ll ever get to knowing how the back would respond on its own. What I’m looking for is a convincing overlap between the two, showing that the back is contributing to the overall sound.

No evidence of a live back here

The outstanding feature of the back response is the peak at 210Hz, and maybe the overall response is higher there as a result. Maybe. The airbody response isn’t present because the soundhole is blocked.

Except for a teensy little peak at 175Hz and a fat-looking feature between 210 and 230Hz, I can’t see any convincing evidence that the back is having any influence on the overall sound. Oh, well.

And for my next trick…

Next I’ll describe making and fitting the top panel, the most crucial element in any guitar.

A strangely painful-sounding title, wouldn’t you say? I’m a bit worried how this will turn out, but I promise you won’t end up with a wedgie or a splint as a result of reading it.

Top and tail blocks

The end blocks hold the two halves of the soundbox together. They are important for the structural integrity of the guitar, particularly the top block that has to absorb the stress of the neck joint under string tension of around 70kg weight. It has to be a substantial chunk of wood accurately fitted to the soundboard, sides and back. That 70kg weight is another reason I like laminated top linings – imagine an adult man standing on the end of the guitar.

I make mine from two blocks of Niugini Rosewood joined at right angles and pinned vertically with two 9mm dowels, all held with Titebond glue. The reason for the shape is that the guitar will have a bolt-on-bolt-off neck, so there needs to be space for horizontal and vertical bolts to land. (In this picture the guitar is resting in sanding dish because I am profiling the bottom edge to the right curvature before fitting the side splints and the linings.)

The front face of the block is shaped to fit by temporarily taking the sides out of the construction mould and sticking some adhesive backed sandpaper where the block will land. The block then gets sanded to an exact fit – and I do the same with the tail block before putting the two half sides back into the mould.

Both blocks then need to have a channel cut into them to fit the 18x6mm top linings. Some careful shaping is needed to get a good fit, because of course the linings have a subtle curve to them. When there’s a good fit I glue them both in place with firm clamping.

The tail block will have to support the strap button and the jack for the pickup, so again I laminate it up out of four 4mm thick pieces with grain crossed to stop it splitting if the guitar is ever dropped on its end.

In this picture you can clearly see the notches in the top lining waiting for the side splints.

The tail wedge

It’s customary to cut away the sides where they join at the tail and replace them with a nicely-contrasting wedge of wood – here I’ve used blackwood. To get a good fit. I make the wedge first, clamp it to the guitar and carefully cut into the sides, and then remove the waste with a chisel. The subtle shape means the when you tap the wedge into place it fits very tightly for a hardly-visible joint.

Profiling the back edge

Once the tail wedge is in and trimmed off, I profile the bottom edge of the soundbox before I glue in the side splints (in all the pictures so far, the guitar is resting in the 20′ radius sanding dish). No matter how hard I try, somehow the sides never match the designed profile and I have to adjust them.

Side splints

The side splints in a guitar are small strips of wood glued vertically to the sides between the soundboard and the back. They reinforce the side panels by crossing the grain to stop splitting, and they add extra rigidity. They are quite important, as Gore and Gilet point out, because when a guitar is played, at the simplest level of analysis the soundboard and the soundbox necessarily move out of phase with each other. The more rigid the soundbox back and sides are – that is, the more the behave like a single unit – the more efficiently the soundboard can vibrate.

So I take my splints seriously now, where before I used to slap in a few little matchsticks made from scrap spruce and move on. I now laminate them from hardwood and notch them carefully into the top and bottom linings. I make them from three 2mm laminations of leftover Blackwood, so they’ll be the same 6mm thickness as the top linings. They’re 9mm wide, and carefully shaped to fit the curvature of the sides.

It’s much easier to fit the splints a bit over-long, then trim them with a saw to match the depth of the side panels – otherwise you’re left sanding end grain when doing the final profiling of the back.

FITTING THE KERFED LININGS

The last task before fitting the back panel is to install the kerfed lings. I fit them by bending a length of the lining around the curve between each side splint, mark the length, and cut the length off with a bandsaw. If they’re cut slightly overlength you can fit them exactly by sanding the ends.

Each piece is glued in, and they need to be clamped. I use spring clothes pegs, with the odd more powerful small spring clamp when needed.

The thing to look for here is that the lining fits well against the sides. Because you’ll be working with the soundbox face down, you can sometimes get an unpleasant shock when it’s all finished – you proudly turn it the right way up and find the top edges of the linings are standing slightly away from the sides.

With a bit more work in the sanding dish the linings and the ends of the side splints will be profiled exactly to mate up with the back plate when it’s ready to fit.

The top linings form the joint between the soundboard and the sides of the guitar. Generally guitar makers use kerfed linings like the one above. They’re easy to bend around the curves, and they act like a series of small reinforcing blocks to tie the top and sides together. They work well, and I use them for the back-to-side joint.

But I have a theory that solid top linings are better at transferring vibrational energy from the soundboard to the soundbox body. They also add greatly to the overall strength and structural rigidity of the guitar. So I laminate them out of thin strips of Australian oak, a wood that takes to heatbending well. In future I think I’ll use bamboo instead, because the trees the “oak” comes from, Eucalyptus regnans or Mountain Ash, are precious. They’re one of the tallest tree species in the world:

I cut the strips on my table saw from a blank that I have thicknessed to 18mm in my drum sander. I then thickness the strips to 2mm in the other dimension (actually, I stay in this dimension to do it) by feeding the cut faces through the drum sander. Using good old Titebond glue, I laminate three 2mm thicknesses in a mould to end up with a pre-shaped lining of 18x6mm.

To make it easier I prebend the strips using the side bender, otherwise they put up a hell of a fight.

Here’s what it looks like out of the mould:

What I’m doing here is cutting notches to house the top of the side-splints that will reinforce the sides. The splints also add rigidity to the sides, and a bit of useful mass as well.

I then glue the linings onto the sides, after which I can saw the ends flush with the construction mould. After that I bolt the two sides of the mould together ready to take the top and tail blocks that unite the sides. Notice the very clever placement of baking paper to avoid gluing the guitar to the mould. Yes, we guitar makers are a cunning lot.

I use a mould and heat blanket to bend my sides rather than try to do it freehand with a heated pipe. Lately I’ve been thinking about trying a temperature-controlled electric bending pipe instead, in the hope (or is it delusion?) that it might give me more control over the final curvature.

When people ask how I bend the sides, I tell them I use a bender. Here’s what I think flashes through their minds before they hit me for being a smart-arse:

No, but seriously folks, people are often overly impressed by bent sides on a guitar. It was the thing that intimidated me the most when I started building, I know, but I’ve found that while there’s a very bountiful source of possible stupid mistakes available to choose from, it’s actually not that hard.

My side bender is a hollow mould on a base, with a waist clamp to draw the wood down into the concave bend and two clamps each at the head and tail to draw the side around the convex curves:

The sides of the mould define the shape, and it’s useful to slightly exaggerate the curves because the bent wood always springs back a bit when it’s removed. Around the edge of the mould you can see the ends of the rods that go from from one side to the other to form the actual bending surface.

The clamps are made from threaded rod and wingnuts from the hardware shop. You can’t see the end clamps that pull the front and back downwards to put the whole piece under tension, but they’re just eyebolts with a wingnut on each that act on the undersurface of the mould base.

There is a thin metal sheet above and below the wood that make up a metal/wood/metal sandwich to stop the grain breaking around the top surface, which is under tension. I use aluminium, though springy stainless steel is better in some ways but very hard to work.

Here are the first two metal/bamboo layers of the sandwich, ready for the blanket and then the second metal sheet on top. I put baking paper in between and give it a squirt of water to help the bend:

The heat is provided by a silicone rubber thermal blanket connected to a temperature controller. In the picture below you can see the thermocouple temperature sensor and the controller that allows heater to work safely and accurately.

Here is what the bent side looked like after the first attempt at 180 degrees C, which is what the manufacturers recommend. You can see that the waist doesn’t conform to the mould shape at all well, so I had to re-bend it several times at higher temperatures before it came right, ending up at 230 degrees:

Here is one bent side clamped into the building half-mould while still warm to allow it to keep the bend:

The bamboo behaved really well during the bending, and showed no sign of delaminating. So far, so good.

In my scheme of things, the next job is to make up the laminated top linings and fit them.

Guitarists have always believed that two identical guitars, each played exclusively by different players with different styles, will end up sounding quite different from each other. Others make sure to keep their guitars in front of their stereo speakers so that whenever they play their favourite music the guitar will vibrate in sympathy and take on the tone of the music it “hears”.

Is there anything in this?

Recently electromagnetic vibrators have been produced to artificially “play in” new instruments. Even I can tell that a brand-new, just strung up guitar sounds pretty raw compared to its sound a few weeks or months later, so I bought a Tonerite vibrator to experiment with.

The Tonerite fits between the strings down near the bridge and has a choice of settings:

When the device is turned on, you can feel the whole instrument vibrate from top to bottom. Tonerite recommend you vibrate for about 3 days for a new instrument, and periodically repeat the treatment to “liven up” older instruments.

But does it do any good?

My trusty tap hammer says that it does, in fact, make a difference. Here is the tonal signature of one of my 12-string guitars before and after the vibration treatment at the very beginning of the instrument’s life:

The blue signature is the instrument’s response before vibrating, and the red afterwards.

It’s pretty clear that the “after” picture is an improvement on the “before”. Entirely new formants (peaks in the response) have formed right across the spectrum from 200 to 1000Hz, and the sensitivity has improved pretty much across the spectrum. My ear agrees that the instrument sounded far richer and livelier after the treatment.

Some caution is needed in looking at the increased response overall, because the traces were produced by two separate and therefore not identical taps, so the “after” trace could have been a heavier tap despite my best intentions.

(This leads to the search for a standard tap device that delivers the same impulse to the guitar every time to make direct comparison more viable – but that’s the subject for another day.)

However, what makes me think that the two are similar enough is that the first peak at about 120Hz is what’s called the coupled Helmholtz response. I wouldn’t expect this to change much after vibration because it is produced by the overall “coupled” response of the top, the air in the soundbox, and the back. This peak is a little higher in the “after” response, which implies that my tap was a bit harder for this one. But the two peaks are similar enough to show that the overall improvement in response after vibration is real, and not just the effect of bumptious tapping.

When you look at tonal signatures such as this, keep in mind that the response (the height of the trace) is measured in deciBels (dB). This can be tricky, because a difference of +10dB actually means a difference in sound power of 10 times. A difference of about 3dB means a doubling of sound power. This means that the red “after” line shows a hugely increased performance across the spectrum.

Why does it work? My belief is that when a guitar is built many localised stress points are set up in its structure. Both the resin glue and the wood will “creep” under the influence of mechanical vibration, smoothing out those localised stresses and improving its ability to respond evenly across the spectrum.

Because it’s not possible to have two identical guitars, it’s hard to say whether different players would have a different final effect – but it’s a reasonable thing to suggest.

In a separate blog I’ll explain what the peaks on a guitar’s tonal signature actually mean. On another I’ll show how my mighty hammer and I produce the traces, with the help of Audacity and Excel.

This is what the outside door to my workshop looks like. The plate was made for my birthday one year by my wife, Wendy – she always gets a laugh when I tell her I’ve just made a new jig to do something or other, and she knows that clamps are always welcome presents as well.

But the truth is that you just can’t make guitars without clamps and jigs. The problem is working out when the jig/clamp population is going to start crowding out everything else in the workshop.

The scary thing is that jigs, which are devices that help produce an accurate shape in some way, always have some kind of clamp on them. For example, what would you call this:

It’s my side bender, so it’s a jig. The white box is the temperature controller for the silicone rubber blanket that supplies the heat. But look at those sneaky clamps to hold the waist and ends down. Clamps are everywhere!

And they come in all different types. Like most luthiers I use go-bars to apply pressure to glue joints. They’re simple and elegant – all you need is an upper and lower rigid surface, then you jam bits of dowel end on between the two.

In this picture I’m joining two panels of Tasmanian Blackwood together for the back plate of a guitar.

I could give you a list of all the types of clamp a luthier needs, but I won’t. Wendy would laugh at me if I did. I plan to show how I build a guitar as these pages relentlessly procreate themselves, so you’ll see them all if you’re interested. But promise you won’t laugh.